Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
328 PM EDT Wed May 01 2024

Valid 00Z Thu May 02 2024 - 00Z Sat May 04 2024

...Severe thunderstorm and flash flooding threat exists across parts of
the central/southern Plains tonight before expanding into the Mississippi
Valley on Thursday...

...Critical Fire Weather potential over portions of the Southern High
Plains through this evening...

...Cool and snowy in parts of the Northwest with above average and
potentially record breaking temperatures into the Ohio Valley and
Mid-Atlantic on Thursday...


A very active start to May is underway throughout much of the central and
southern Plains as scattered to numerous thunderstorms developing this
afternoon are forecast to continue into the overnight hours. Low pressure
strengthening over the central High Plains and an attached warm front
extending through the mid-Mississippi Valley will help draw ample moisture
northward from the Gulf of Mexico. Meanwhile, convection forming off a
southern High Plains dryline should push eastward across parts of Oklahoma
and Texas, with several storms potentially containing very heavy rainfall
and training over similar regions. The flash flooding risk is highest
across parts of central and eastern Texas through tonight, where a
Moderate Risk (level 3/4) of Excessive Rainfall is in effect. Several
inches of rain falling over sensitive and previously saturated terrain
could lead to numerous flash floods, with a few significant flooding
events possible. Additionally, a Slight Risk (level 2/4) of Excessive
Rainfall extends northward into central Oklahoma and all of north-central
Texas, with a separate Slight Risk in effect from north-central Kansas to
western Iowa. These regions can also expect scattered thunderstorms
containing intense rainfall rates and possible instances of flash flooding
through early Thursday. These evening and overnight thunderstorms are also
expected to contain damaging wind gusts, large hail, and a few tornadoes
from West Texas to south-central Kansas. These regions fall within an
Enhanced Risk (level 3/5) of severe thunderstorms issued by the Storm
Prediction Center. As the low pressure system and attached cold front
gradually swings eastward into the Upper Midwest and Mississippi Valley on
Thursday, showers and thunderstorms are also expected to expand eastward.
Scattered severe thunderstorms and flash flooding are possible between a
large area stretching from the Midwest to southern Plains and Lower
Mississippi Valley, with the threat becoming more isolated across the
Plains and Deep South on Friday. Residents and visitors are reminded to
have multiple ways to receive warnings and never drive across flooding
roadways.

One additional hazard in the central U.S. due to the potent weather system
impacting the region will be the increased fire weather concerns over the
southern High Plains through this evening. Low relative humidity combined
with gusty winds and dry terrain could cause wildfires to develop easily
and spread rapidly. Red Flag Warnings are in effect and stretch from
eastern New Mexico to southeast Colorado, far west Oklahoma, and the
northwest Texas Panhandle.

Elsewhere, a weather pattern consisting of upper-level riding over the
East and troughing over the Northwest will support cooler weather in the
northern Rockies and Northwest with summer-like warmth spreading from
parts of the Midwest to the Mid-Atlantic. In addition to the below average
temperatures in the Northwest, snow is likely across the high terrain of
the northern Rockies through Friday. Meanwhile, the above average warmth
throughout the Midwest, Ohio Valley, and Mid-Atlantic could break daily
high temperature records on Thursday as highs reach into the upper 80s and
low 90s. New England will remain cooler, however, through the end of the
week as high pressure noses southward along the coast and promotes
easterly flow off the chilly Atlantic.


Snell


Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Wed May 1 2024

Valid 12Z Sat May 04 2024 - 12Z Wed May 08 2024


...Renewed heavy rain threat in the south-central U.S. this
weekend...


...Overview...

An upper low that is strong for this time of year is forecast to
move into the West this weekend and track east into the Plains as
next week progresses. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West. Ahead of this, a subtropical jet coming
into the south-central U.S. and the western side of a front
stalling will lead to rain and thunderstorms in the southern half
of the Plains and Mississippi Valley this weekend, potentially in
some areas that have received ample rainfall recently and/or will
during the short range period. Rain and storms are also possible
farther east and to the north/west associated with a strong low
pressure system across the northern U.S.. Then mean upper ridging
downstream will lead to warming temperatures in much of the central
and eastern U.S. into next week.


...Guidance/Predictability Assessment...

Main feature of concern during the period will be with the upper
low that drops through the east Pacific and enters the West this
weekend, shifting east and northward as it strengthens again over
the northern Plains early next week. Compared to recent runs,
models have trended more agreeable in terms of track and strength
with this system, all suggesting a reasonably deep upper and
surface low early next week over the Northern Plains. Models also
trended towards heavier QPF across the northern High Plains region
as well and that is reflected in todays issuance of the WPC QPF. A
general model compromise between the GFS, ECMWF, CMC, and UKMET
worked well for the Saturday-Monday period.

By Tuesday and Wednesday, there is greater uncertainty in the
exact track of the low, depending on blocky downstream ridging
across the East. Latest 12z guidance today is trending blockier,
forcing the upper low more northward than previous guidance which
brought it more eastward into the Upper Midwest/Great Lakes.
Additionally, upstream, another round of energy and troughing may
dive through the Pacific Monday-Wednesday and whether energy makes
it far enough south to impact the Northwest U.S.. The Canadian
model (especially the new 12z run) continues to be most
aggressive/south with this energy compared to the GFS/ECMWF. The
Tuesday-Wednesday blend for next week trended towards the ensemble
means along with smaller contributions from the GFS/ECMWF amidst
growing forecast uncertainty in the late periods. Overall,
maintained reasonable continuity with the previous WPC shift
besides a much stronger northern Plains surface low.


...Weather/Hazards Highlights...

As the upper low tracks into and through the West, widespread
precipitation is likely. Moisture streaming perpendicular into the
foothills of the Sierra Nevada could cause a heavy rain and
flooding threat there, and a Marginal Risk remains in place for Day
4/Saturday. Meanwhile the higher elevations of the Sierra, north
into the Cascades, and east into the Intermountain West/Rockies
will see some May snow. Precipitation is forecast to move steadily
eastward through the weekend into early next week, reaching the
northern Plains to Midwest Monday and Tuesday. Rainfall on the west
side of the strong surface low could be heavy across eastern
Montana into the Dakotas. The trough/low aloft and frontal
boundaries could also produce some high winds across the Great
Basin to Rockies.

Meanwhile this weekend, the subtropical jet looks to be active
coming into the south-central U.S., focusing convection in a moist
and unstable environment. Many areas in the south-central U.S. will
have quite saturated antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. On Saturday and Sunday, Marginal Risks are delineated in
the ERO across southern/central portions of the Plains and
Mississippi Valley. Will also maintain an embedded Slight Risk on
Day 4/Saturday for portions of southwest Oklahoma and into portions
of northwest Texas where potential exists for an MCS with heavier
rainfall rates, though the position varies somewhat between models.
Farther east on Saturday, an axis of instability is forecast
across the southern/central Appalachians and into the Piedmont
between the synoptic cold front and backdoor front in the East as a
shortwave aloft moves through. These ingredients could produce
heavy rainfall and a Marginal Risk is in place there for Day 4 as
well. Showers and thunderstorms should continue for parts of the
eastern U.S. on Sunday.

For the weekend, portions of the east-central U.S. are forecast to
see above average temperatures, but the Northeast and Mid-Atlantic
could be cooler than average at least for highs due to a backdoor
front. Temperatures this weekend should warm in the Intermountain
West to northern Plains ahead of the West upper low, while cooler
than normal temperatures by 10-20 degrees for highs will impact the
West. As the upper low/trough shifts east, this should cool down
temperatures toward the north-central U.S. while staying below
average in the Northwest. On the other hand, mean upper ridging
will cause warm temperatures to spread across the south-central and
east-central U.S. into the Eastern Seaboard. 90s are forecast to
become more widespread across the southern tier, with temperatures
over 100F (and even higher heat indices) into southwest Texas and
possibility for the first excessive heat days of the year.


Santorelli/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
335 AM EDT Wed May 01 2024

Valid 00Z Thu May 02 2024 - 00Z Thu May 09 2024

Trade winds will be transitioning to a more easterly direction and
gradually strengthening on Wednesday as surface highs develop well
north of the state. The highs are likely to consolidate and
strengthen over the next few days, which will in turn cause trades
to strengthen to eventually breezy levels. Meanwhile model
guidance is in fairly good agreement with showing a mid-upper
trough/low centered west of Hawaii late week and then coming
overhead by early next week. This could provide some instability
for increased showers, which would favor windward and mauka areas
given the increasing trades. Rainfall totals do not appear to be
significant though. This wet trade wind pattern looks to continue
into midweek next week.

Tate

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Wed May 1 2024
Day 1
Valid 01Z Thu May 02 2024 - 12Z Thu May 02 2024

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
CENTRAL AND EASTERN TEXAS...

...16Z Update...

...Southern Plains...

Latest CAMs have shown a growing consensus on the potential for
training cells farther west towards the Edwards Plateau closer to
the dryline. Latest RAP guidance shows an acceleration in the 850mb
LLJ over the Rio Grande this afternoon that provides strong
theta-e advection at that level. This also coincides with the
region being ideally placed beneath the left-exit region of a 250mb
subtropical jet streak, sufficient shear for sustaining
mesocyclones, and PWs >1" that are above the 90th climatological
percentile. With some portions of the Edwards Plataea and Concho
Valley having received as much as 300% of normal rainfall over the
past week, soils are a little more susceptible to runoff. Decided
to expand the Slight Risk into this region for these reasons.

In South Central TX, latest CAMs consensus shows the QPF footprint
shifting south towards the I-35 and I-10 corridors between San
Antonio and Austin, and even on west into the heart of the Hill
Country. This is largely due to the core of the 850mb LLJ being
focused more over this part of the region in recent runs. These
areas are more elevated and can further increase low-level ascent
in a synoptic scale setup that supports large scale growth of
organized thunderstorm clusters. The 12Z RAP depicts highly
saturated surface-700mb soundings north and east of San Antonio
this evening where low-mid level RH values are averaging around 90%
saturation and warm cloud layers as deep as 13,000ft. The ECMWF
SAT shows PWs approaching 1.75" that are as high as the 99.5
climatological percentile by 00Z this evening and MUCAPE >2,000
J/kg. The 12Z HREF now sports moderate chances (40-60%) for
rainfall totals >5" near San Antonio and suburbs of the city along
I-35 and I-10. With the added concern for 2-3"/hr rainfall rates
within the more urbanized I-35 corridor, where runoff will be
accentuated, chose to expand the Moderate Risk area west the
Austin/San Antonio metro areas.

...Midwest/Central Plains...

The northern flank of the Slight and Marginal Risks were trimmed
due to less available instability over southeast SD, northwest IA,
and central NE. The Slight Risk was expanded farther south into
northern KS and northwest MO where these areas are more ideally
placed at the nose of a robust 850mb LLJ that tops the 90th
climatological percentile for wind speeds over eastern KS.
Admittedly, there remains quite the dispersion in CAMs guidance in
the placement of heaviest rainfall some some guidance as far north
as east-central NE and as far south as east-central KS. Using the
position of the 850mb LLJ and the expected frontal position as
references, opted to position the Slight just north of the frontal
boundary over central Kansas. This should be where the strongest
ascent occurs as the 850mb LLJ intersects the front and where MUCAPE
>1,000 J/kg is also likely to be in place tonight.

Mullinax


---Previous Discussion---

...Southern Plains...

Latest hi-res ensemble and associated CAMs have come into better
agreement on the development of a complex across TX with origins
out in west TX near the dryline, growing upscale through the
evening with aid from a well defined LLJ and shortwave progressive
within the sub-tropical jet over the region. Given consensus now
favoring at least 2" of precip over a large expanse of central and
eastern TX with much higher embedded amounts given the convective
nature of the precip and anomalous tropospheric moisture (2-2.5
standard deviations above normal PWATs), have allowed for an
upgrade to a Moderate Risk over the aforementioned areas. This is
now the primary period of interest with a shift away from the
previous MDT that was forecast for the now D2 time frame.

The reasoning for the shift was due to the progressive nature of
the convective pattern within the latest CAMs suite, as well as
some instance within the ensemble bias corrected mean QPF
footprint. Previous forecast iterations were more robust for the
now D2 time frame, but the addition of CAMs guidance and the
ensemble have allowed for a shift in the timing of when the
heaviest precipitation would occur, as well as the general
locations. 00z HREF EAS probabilities for at least 2" of rainfall
are now upwards of 40% across a large swath of central and east TX,
a good portion now overlapping with the area that saw extreme
rainfall totals over 6" from this past weekend. The conjunction of
heavy rain over those areas impacted and forecasted rates exceeding
2"/hr will likely force easier flash flooding potential due to the
moist antecedent conditions in place. Also of note is the area
over central TX where despite missing the previous rainfall, the
flash instances are climo favored due to the soils and high runoff
potential. With coordination with the local WFO's involved within
both TX and LA, the MDT risk was shifted to account for the abrupt
change in convective timing and impacts.


...Midwest/Central Plains...

A closed upper low is currently analyzed over Big Sky into the
southwestern Canadian Provinces. A robust mid-level vorticity
maxima will pivot around the base of the mean trough carved out by
the upper low with sights on the central plains and adjacent
Midwest allowing for rapid convective development downstream with a
corridor of heavy rain forming later this afternoon through the
remainder of the period. Guidance has trended to a favored heavy
rain footprint across northern KS through eastern NE with the
heaviest rain with a swath of 2-4" of rainfall over the course of
this evening into early Thursday AM. 00z HREF EAS probabilities for
at least 1" have become most aggressive in-of southeast NE which
matches well with the current ML output from the past several runs.
This makes sense synoptically given the best mid-level forcing
will be downstream of the approaching vort max in an axis of
diffluence perfectly aligned overhead. Rates between 1-2"/hr will
be common within the areal extent of convection leading to some
local totals between that 2-4" marker forecast. This should be
sufficient for some higher flash flood potential within larger
urbanized settings and areas that see the higher rates in question.
The progressive nature of the precip will limit the higher end
impacts, so the previous SLGT risk forecast will suffice despite
the alignment synoptically.

Kleebauer

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Wed May 1 2024
Day 2
Valid 12Z Thu May 02 2024 - 12Z Fri May 03 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF THE
MISSISSIPPI VALLEY EXTENDING FROM THE NORTHERN MIDWEST DOWN TOWARDS
THE GULF COAST...

...2030Z Update...

...Midwest...

Trimmed down the far northern and western flanks of the Marginal
and Slight in the Midwest where probabilistic guidance in heavier
QPF totals >1" were decreasing and instability will be lower to the
north of the surface low track through the Upper Mississippi
Valley Thursday evening. The area most likely to see the heaviest
rainfall will be closer to the triple point low, which in this case
would favor an area from southern IA on north and east through
eastern IA and into southwest WI. NAEFS shows this designated area
sporting >90th climatological percentile PWs (1.3-1.5") between 18Z
Thurs and 00Z Fri, as well as IVTs above the 97.5 climatological
percentile (500-700 kg/m/s) in that same period. 12Z HREF shows
MUCAPE between 500-1,000 J/kg will also be present. Hourly rainfall
rates up to 2"/hr are possible in the strong est storms, which
would support the potential for slash flooding in these areas.
Farther south, a negatively tilted 500-700mb axis emerging out of
the Ozarks will still be capable of producing strong-to-severe
thunderstorms that could track across much of western MO and
eastern KS. Localized rainfall rates surpassing 2"/hr cannot be
ruled out given PWs topping 1.5", MUCAPE approaching 1,500 J/kg,
and mean 1000-500mb RH values as high as 90%.

...Southern Plains and Lower Mississippi Valley...

The expansive Slight Risk in the South Central U.S. was expanded a
little farther west more into central TX and central OK where
latest CAMs guidance has slowed down the cold front approaching
from the northwest. Farther south, there remains some uncertainty
on the position and duration of the ongoing cluster of storms in
the early-mid morning hours Thursday. Should storms stick around
longer into the morning hours along and north of I-10, there could
be the need for either an additional expansion of the Slight Risk
or an upgrade to Moderate Risk should flash flooding be more
extensive and linger beyond 12Z Thursday. Recent rainfall in
western Louisiana (2-5" above normal over the past 7 days) has left
soils in the area more sensitive to 2-3"/hr rainfall rates, which
are supported with PWs just short of 2.0" and skinny MLCAPE up to
1,000 J/kg present. Overall, the Slight Risk remains on track but
the setup in eastern TX and western LA will be closely monitored
overnight.


Mullinax


---Previous Discussion---

...Midwest...

Previous forecast still has merit with little deviation from past
issuance. Thus, maintained continuity from the last discussion with
some minor changes to details within the discussion below....

Overnight convection from this evening will continue to march
northeastward as the congealed cold pools from the initial cell
mergers will advance through the central Midwest with sights on
northern IA into WI and MN on Thursday morning. The complex will
eventually peter out as it moves northward into a less favorable
environment which will put an end to the flooding threat as we move
into the afternoon. Later in afternoon and evening, a cold front
will propagate to the east out of the central plains with a
redevelopment of convection over eastern KS, IA, and northwest MO.
This will allow for the secondary peak of flash flooding potential
as thunderstorms migrate through the mid and upper Mississippi
Valley, impacting some areas that were just affected in the last 24
hrs. With the lower FFGs expected, the threshold for flooding will
be lower than climatological norms for some areas within the
expected secondary convective regime. Totals are currently within
the 1.5-3" range on guidance which is significant enough to
warrant at least the current SLGT risk.

...Southern Plains and Lower Mississippi Valley...

As stated previously in the D1 outlook, our complex of
thunderstorms in TX will migrate eastward into the Lower
Mississippi Valley with heavy rainfall likely within the western
half of LA into southern AR. The setup has now trended towards a
"front loaded" forecast where the primary rainfall in the period
that would induce flash flood concerns will occur during Thursday
morning and early afternoon before dissipating due to the loss of
sufficient upper forcing. The back end of the latest CAMs and
associated HREF signal the opportunity for upwards of 2-3" of
rainfall during that short time frame Thursday AM which will be
enough to cause problems given the overlap from the previous
weekend event that dropped copious amounts of rainfall over
southeast TX into western LA. 00z HREF blended mean has an areal
extent of 1.5-2" with probability matched mean upwards of 3" along
the TX/LA border. HREF neighborhood probability for rates exceeding
2"/hr are also upwards of 30-40% within the same areas, correlating
with the forecasted rainfall in that 2-3" zone. The setup really
comes down to the timing of the complex in TX as to whether a
reintroduction of a MDT risk will be needed after it was downgraded
and shifted one period earlier. This will be assessed in future
forecast updates as we closely monitor radar and hi-res guidance
trends over the course of today and overnight into Thursday. For
now, the SLGT risk will remain with wording of it being on the
higher end of the risk threshold.

Further north across KS and OK, the cold front to the north will
move south with a push from high pressure nosing in through the
plains in wake of the low pressure moving to the north. An area of
convection will likely form over the central plains and move south
with the boundary leading to more locally heavy rainfall in
association with the area of convection. The threat is less
aggressive compared to what is expected further south, but the
chances are still prevalent. The SLGT risk was maintained from the
previous D3 connecting the two primary areas of interest to the
north and south.

Kleebauer

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
509 PM EDT Wed May 1 2024
Day 3
Valid 12Z Fri May 03 2024 - 12Z Sat May 04 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF
TEXAS, THE LOWER MISSISSIPPI VALLEY, TENNESSEE VALLEY, CENTRAL
PLAINS, AND THE PACIFIC NORTHWEST...

...2030Z Update...

Little in the way of change for the inherited Marginal Risks aside
from minor adjustments based on the latest QPF. There was some
consideration to adding a Marginal Risk in portions of the Great
Lakes given the >90th climatological percentile PWs present
according to the ECMWF SAT, but thunderstorm activity looks fairly
progressive and confidence in sufficient instability being present
was not high as of this forecast cycle.

Mullinax


---Previous Discussion---

...Southern Plains and Southeast...

A multitude of shortwaves will ripple through the flow out of the
subtropical jet with convective development across portions of TX
through the Lower Mississippi Valley up into the Tennessee Valley.
The convection over TX will have the best potential for flash flood
concerns due to the regional instability and alignment of a stalled
frontal boundary forecast across the central and eastern portion of
the state with the dryline positioned over west TX. QPF
distribution is scattered in nature with the maxima shifting all
over the place pending deterministic. Considering the spread, the
MRGL risk inherited was maintained, but there is potential for an
upgrade within the two area boundaries due to the features being a
focal point for training convection and surface convergence.

Further east, widely scattered thunderstorms will produce heavy
rainfall at times with the best opportunity occurring earlier in
the period when the strongest shortwave progresses through the
region(s). Ensemble bias corrected QPF is consistent with a maximum
across LA which will have gone through some significant precip
prior, so that will be another area to monitor in time for
potential upgrades. Forecast QPF is still marginal for any upgrade
potential, but CAMs will shed more light as we move closer to the
time period of interest.

...Central Plains...

A fairly robust mid-level perturbation will eject east-southeast
out of WY/CO with convection initiating over the Front Range,
eventually congealing into a MCS as it migrates eastward through
southern NE into KS. Models are in agreement of at least some
modest QPF with 1-2" scattered across the aforementioned areas and
QPF maxima running upwards of 3" at this time. These MCS setups are
notorious for localized flooding concerns, so wanted to make sure
to add a Marginal Risk area to cover for the potential. As we move
closer, the MRGL risk will likely be shifted around to account for
the trends in guidance, especially within D1 when the CAMs and HREF
provide more favor.

...Pacific Northwest...

A relatively weak IVT pulse will enter coastal OR and northwest CA
by the second half of the period with locally heavy rainfall
expected through Saturday morning. IVT index is running at an "AR
1" which signals a weaker atmospheric river event, but one that can
still produce modest rainfall totals to the coastal plain. Ensemble
trends have come up a bit from previous forecasts with QPF running
between 1.5-2.5 over the span of 12 hrs (00-12z Saturday), with a
bit more as we head into D4. As a result, kept continuity with the
MRGL risk from previous forecast package.

Kleebauer

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Wed May 1 2024


As the upper low tracks into and through the West, widespread
precipitation is likely. Moisture streaming perpendicular into the
foothills of the Sierra Nevada could cause a heavy rain and
flooding threat there, and a Marginal Risk remains in place for Day
4/Saturday. Meanwhile the higher elevations of the Sierra, north
into the Cascades, and east into the Intermountain West/Rockies
will see some May snow. Precipitation is forecast to move steadily
eastward through the weekend into early next week, reaching the
northern Plains to Midwest Monday and Tuesday. Rainfall on the west
side of the strong surface low could be heavy across eastern
Montana into the Dakotas. The trough/low aloft and frontal
boundaries could also produce some high winds across the Great
Basin to Rockies.

Meanwhile this weekend, the subtropical jet looks to be active
coming into the south-central U.S., focusing convection in a moist
and unstable environment. Many areas in the south-central U.S. will
have quite saturated antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. On Saturday and Sunday, Marginal Risks are delineated in
the ERO across southern/central portions of the Plains and
Mississippi Valley. Will also maintain an embedded Slight Risk on
Day 4/Saturday for portions of southwest Oklahoma and into portions
of northwest Texas where potential exists for an MCS with heavier
rainfall rates, though the position varies somewhat between models.
Farther east on Saturday, an axis of instability is forecast
across the southern/central Appalachians and into the Piedmont
between the synoptic cold front and backdoor front in the East as a
shortwave aloft moves through. These ingredients could produce
heavy rainfall and a Marginal Risk is in place there for Day 4 as
well. Showers and thunderstorms should continue for parts of the
eastern U.S. on Sunday.

For the weekend, portions of the east-central U.S. are forecast to
see above average temperatures, but the Northeast and Mid-Atlantic
could be cooler than average at least for highs due to a backdoor
front. Temperatures this weekend should warm in the Intermountain
West to northern Plains ahead of the West upper low, while cooler
than normal temperatures by 10-20 degrees for highs will impact the
West. As the upper low/trough shifts east, this should cool down
temperatures toward the north-central U.S. while staying below
average in the Northwest. On the other hand, mean upper ridging
will cause warm temperatures to spread across the south-central and
east-central U.S. into the Eastern Seaboard. 90s are forecast to
become more widespread across the southern tier, with temperatures
over 100F (and even higher heat indices) into southwest Texas and
possibility for the first excessive heat days of the year.


Santorelli/Tate

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Wed May 1 2024


As the upper low tracks into and through the West, widespread
precipitation is likely. Moisture streaming perpendicular into the
foothills of the Sierra Nevada could cause a heavy rain and
flooding threat there, and a Marginal Risk remains in place for Day
4/Saturday. Meanwhile the higher elevations of the Sierra, north
into the Cascades, and east into the Intermountain West/Rockies
will see some May snow. Precipitation is forecast to move steadily
eastward through the weekend into early next week, reaching the
northern Plains to Midwest Monday and Tuesday. Rainfall on the west
side of the strong surface low could be heavy across eastern
Montana into the Dakotas. The trough/low aloft and frontal
boundaries could also produce some high winds across the Great
Basin to Rockies.

Meanwhile this weekend, the subtropical jet looks to be active
coming into the south-central U.S., focusing convection in a moist
and unstable environment. Many areas in the south-central U.S. will
have quite saturated antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. On Saturday and Sunday, Marginal Risks are delineated in
the ERO across southern/central portions of the Plains and
Mississippi Valley. Will also maintain an embedded Slight Risk on
Day 4/Saturday for portions of southwest Oklahoma and into portions
of northwest Texas where potential exists for an MCS with heavier
rainfall rates, though the position varies somewhat between models.
Farther east on Saturday, an axis of instability is forecast
across the southern/central Appalachians and into the Piedmont
between the synoptic cold front and backdoor front in the East as a
shortwave aloft moves through. These ingredients could produce
heavy rainfall and a Marginal Risk is in place there for Day 4 as
well. Showers and thunderstorms should continue for parts of the
eastern U.S. on Sunday.

For the weekend, portions of the east-central U.S. are forecast to
see above average temperatures, but the Northeast and Mid-Atlantic
could be cooler than average at least for highs due to a backdoor
front. Temperatures this weekend should warm in the Intermountain
West to northern Plains ahead of the West upper low, while cooler
than normal temperatures by 10-20 degrees for highs will impact the
West. As the upper low/trough shifts east, this should cool down
temperatures toward the north-central U.S. while staying below
average in the Northwest. On the other hand, mean upper ridging
will cause warm temperatures to spread across the south-central and
east-central U.S. into the Eastern Seaboard. 90s are forecast to
become more widespread across the southern tier, with temperatures
over 100F (and even higher heat indices) into southwest Texas and
possibility for the first excessive heat days of the year.


Santorelli/Tate

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Wed May 1 2024

Valid 00Z Thu May 02 2024 - 00Z Sun May 05 2024

...Northern Rockies and Oregon Cascades...
Day 1...

Mid-level low pressure remains centered over southern Saskatchewan
through tonight while a reinforcing trough rounds the low, crossing
from the ID/WY border this afternoon and becoming the dominant low
on Friday north of Minnesota. A second reinforcing trough from the
northeast Pacific crosses the PacNW coast tonight.

Flow wrapping around this low pushes into the eastern slopes of the
northwest MT Rockies/around Glacier NP this evening, shifting
southeast over the west-central MT ranges (like the Big and Little
Belts) late tonight that continues through Friday morning. Snow
levels through Day 1 are generally 4000 ft over this area. Day 1
PWPF for >6" are 50-80% for the Lewis, Mission, and Big/Little
Belts. 12Z HREF mean snow rates generally peak around 1"/hr with
this generally moderate precipitation.

The wave reaching the PacNW coast tonight brings a swath of late
night locally moderate precip along the Oregon Cascades with snow
levels ranging from 4500ft in the northern OR Cascades to 6000ft in
the southern. Day 1 PWPF for >6" is generally 40-80% for the higher
Cascades.


...Cascades through Sierra Nevada...
Day 3...

A deep, closed low tracks south of the Gulf of Alaska Thursday and
approaches the PacNW coast late Friday then tracks over the OR/CA
coast Saturday. Snow levels will drop from around 7000ft with the
track of this low. The low track has been rather uncertain in
recent days, but with the 12Z global model suite, there is decent
certainty with the mid-level low center tracking into northern NV
through Saturday night. Snow levels of generally 5000-6000ft can be
expected over the Cascades/Sierra Nevada during the heaviest precip
Friday night/Saturday. Day 3 PWPF for >6" are generally 40-70% for
the southern OR Cascades/CA Cascades/Shasta-Siskiyou and
northern/central Sierra Nevada. Onshore flow would continue through
Saturday night, maintaining decent snow rates through that time for
similar areas.


The probability of significant icing is less than 10 percent.


Jackson

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
248 PM EDT Wed May 1 2024

Valid 12Z Sat May 04 2024 - 12Z Wed May 08 2024


...Renewed heavy rain threat in the south-central U.S. this
weekend...


...Overview...

An upper low that is strong for this time of year is forecast to
move into the West this weekend and track east into the Plains as
next week progresses. This will support cooler than normal
temperatures, gusty winds, and precipitation including higher
elevation snow in the West. Ahead of this, a subtropical jet coming
into the south-central U.S. and the western side of a front
stalling will lead to rain and thunderstorms in the southern half
of the Plains and Mississippi Valley this weekend, potentially in
some areas that have received ample rainfall recently and/or will
during the short range period. Rain and storms are also possible
farther east and to the north/west associated with a strong low
pressure system across the northern U.S.. Then mean upper ridging
downstream will lead to warming temperatures in much of the central
and eastern U.S. into next week.


...Guidance/Predictability Assessment...

Main feature of concern during the period will be with the upper
low that drops through the east Pacific and enters the West this
weekend, shifting east and northward as it strengthens again over
the northern Plains early next week. Compared to recent runs,
models have trended more agreeable in terms of track and strength
with this system, all suggesting a reasonably deep upper and
surface low early next week over the Northern Plains. Models also
trended towards heavier QPF across the northern High Plains region
as well and that is reflected in todays issuance of the WPC QPF. A
general model compromise between the GFS, ECMWF, CMC, and UKMET
worked well for the Saturday-Monday period.

By Tuesday and Wednesday, there is greater uncertainty in the
exact track of the low, depending on blocky downstream ridging
across the East. Latest 12z guidance today is trending blockier,
forcing the upper low more northward than previous guidance which
brought it more eastward into the Upper Midwest/Great Lakes.
Additionally, upstream, another round of energy and troughing may
dive through the Pacific Monday-Wednesday and whether energy makes
it far enough south to impact the Northwest U.S.. The Canadian
model (especially the new 12z run) continues to be most
aggressive/south with this energy compared to the GFS/ECMWF. The
Tuesday-Wednesday blend for next week trended towards the ensemble
means along with smaller contributions from the GFS/ECMWF amidst
growing forecast uncertainty in the late periods. Overall,
maintained reasonable continuity with the previous WPC shift
besides a much stronger northern Plains surface low.


...Weather/Hazards Highlights...

As the upper low tracks into and through the West, widespread
precipitation is likely. Moisture streaming perpendicular into the
foothills of the Sierra Nevada could cause a heavy rain and
flooding threat there, and a Marginal Risk remains in place for Day
4/Saturday. Meanwhile the higher elevations of the Sierra, north
into the Cascades, and east into the Intermountain West/Rockies
will see some May snow. Precipitation is forecast to move steadily
eastward through the weekend into early next week, reaching the
northern Plains to Midwest Monday and Tuesday. Rainfall on the west
side of the strong surface low could be heavy across eastern
Montana into the Dakotas. The trough/low aloft and frontal
boundaries could also produce some high winds across the Great
Basin to Rockies.

Meanwhile this weekend, the subtropical jet looks to be active
coming into the south-central U.S., focusing convection in a moist
and unstable environment. Many areas in the south-central U.S. will
have quite saturated antecedent conditions because of recent and
upcoming short range events, so this will be a factor for flooding
as well. On Saturday and Sunday, Marginal Risks are delineated in
the ERO across southern/central portions of the Plains and
Mississippi Valley. Will also maintain an embedded Slight Risk on
Day 4/Saturday for portions of southwest Oklahoma and into portions
of northwest Texas where potential exists for an MCS with heavier
rainfall rates, though the position varies somewhat between models.
Farther east on Saturday, an axis of instability is forecast
across the southern/central Appalachians and into the Piedmont
between the synoptic cold front and backdoor front in the East as a
shortwave aloft moves through. These ingredients could produce
heavy rainfall and a Marginal Risk is in place there for Day 4 as
well. Showers and thunderstorms should continue for parts of the
eastern U.S. on Sunday.

For the weekend, portions of the east-central U.S. are forecast to
see above average temperatures, but the Northeast and Mid-Atlantic
could be cooler than average at least for highs due to a backdoor
front. Temperatures this weekend should warm in the Intermountain
West to northern Plains ahead of the West upper low, while cooler
than normal temperatures by 10-20 degrees for highs will impact the
West. As the upper low/trough shifts east, this should cool down
temperatures toward the north-central U.S. while staying below
average in the Northwest. On the other hand, mean upper ridging
will cause warm temperatures to spread across the south-central and
east-central U.S. into the Eastern Seaboard. 90s are forecast to
become more widespread across the southern tier, with temperatures
over 100F (and even higher heat indices) into southwest Texas and
possibility for the first excessive heat days of the year.


Santorelli/Tate


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw